*{
    margin: 0;
}
.beijing{
    position: fixed;
    top: 0;
    left: 0;
    bottom: 0;
    right: 0;
    background-color: gray;
    opacity: 0.7;
}
.tankuang{
    text-align: center;
    font-size: 20px;
    position: fixed;
    background-color: wheat;
    height: 300px;
    width: 500px;
    left: 50%;
    top: 50%;
    margin-left: -200px;
    margin-top: -200px;
}
.hide{
    display: none;
}
